As described herein, the fluid barrier of the invention is adaptable for use in a variety of applications to prevent passage of selected "fluids", defined herein to include both liquids and gasses, between a first location and a second location a distance removed therefrom by interposing the barrier therebetween. One preferred use for the barrier member of the invention is as a geosynthetic clay liner for use in isolating leachate from a waste containment facility, e.g., a landfill, from adjacent groundwater systems. The applicability of the invention is not, however, limited solely to use in waste containment facilities. Rather, the presently described fluid barrier member is useful in any situation where it is desirable to selectively hinder or prevent the flow of one or more fluids from a first location to a second location a distance removed therefrom, particularly wherein the intervening surface between the first and second locations is sloped or otherwise uneven. Such applications include, but are not limited to stabilizing the subsurface soil supporting a roadway or a railroad track, constructing a water retaining structure such as an earthen dam or a canal, containing leakage from a pipeline, liquid containment facility, or storage tank, minimizing leakage into underground structures such as tunnels, mines, or retaining structures, insulating structures from hot or cold weather or subsurface water attributable to the ground-water table or capillary migration, and isolating the subterranean portion of buildings likely to be affected by seismic forces. For convenience in explaining the invention however, it will be mainly described herein with relation to its use as a waste containment facility liner or cover system with the understanding that such use is not limiting.
Waste containment facilities, such as landfills, are ordinarily provided with a low hydraulic conductivity barrier and drainage system comprising a liner formed of compacted clay or a layer of water swellable clay overlain by one or more sheets of geosynthetic material, e.g., a geomembrane and a geotextile. Such liners are typically installed to isolate the leachate produced by the waste containment facility from adjacent groundwater systems. In addition, these facilities are also required to be covered with a low hydraulic conductivity barrier and drainage system upon closure. In the United States liners and covers are, in fact, required for use in all hazardous waste and new or expanded municipal solid waste containment facilities under subtitles C and D of the Federal Resource and Conservation Recovery Act (1976).
The static and dynamic (e.g., seismic activity) stability of such liner and cover systems is controlled by their shear strength, as measured at the component mid-plane or interfaces. Liner stability is of critical importance for preventing liner failure and release of leachate, particularly when the topographical surface of the waste containment facility site is not substantially level, i.e., wherein the surface of the facility slopes at a relatively substantial angle, i.e., of greater than about 9-10 degrees. Cover stability is also of critical importance for preventing a slope stability failure that may endanger people or property and allow precipitation to infiltrate the waste.
In the earliest prior art, waste containment facility liners were formed by applying several feet of barrier material, such as natural soil or a mixture of natural soil and bentonite, directly to the soil surface of the facility. The barrier material was thereafter impacted into place and covered by a layer of soil. Thereafter however, a composite liner was developed. These articles comprise a compacted clay liner overlain by a geomembrane. This dual component liner system was found to be useful for providing multiple protection against leakage of leachate from waste containment facilities.
There are several major problems associated with the placement and use of the compacted clay liners described above. These include the difficulty and expense of locating and transporting a suitable type and quantity of "borrow material", i.e., a term used in the art to describe soil which is used to construct the compacted clay layer in forming the liquid barrier; desiccation cracking in arid climates, freeze-thaw cracking in cold climates and saturation or excessive water content in humid climates. In addition, extremely expensive field test sections and field hydraulic conductivity tests must be conducted to verify that the hydraulic conductivity is within the limits required under the applicable regulations, i.e., hydraulic conductivity less than 10.sup.-7 cm/second. The liner thus produced ranges up to about 3 feet in thickness and costs from about $3 to $10 per square foot to manufacture.
As noted above the compacted clay must, under the applicable regulations, exhibit a hydraulic conductivity of less than 10.sup.-7 cm/second. The hydraulic conductivity of the compacted clay is, however, extremely sensitive to a number of liner construction parameters, including but not limited to the compaction water content, dry unit weight, the type of compaction equipment used, compactive effort and number of compactor passes.
In general, however, increasing the compaction water content leads to a diminution in the hydraulic conductivity of the barrier, as well as the strength of the interface between the compacted clay and the geomembrane. Therefore, a compromise between minimizing the hydraulic conductivity versus maximizing the interface strength or stability is sought. This requires, however, that during its construction, the liner must be limited to a narrow range of compaction water content and dry unit weight. This range is extremely difficult and expensive to achieve and maintain. The problems associated with compacted clay liners are similar to those associated with compacted clay covers.
In an effort to overcome the drawbacks described above with compacted clay liners and covers, prefabricated geosynthetic clay liners, e.g., bentonite mats, prefabricated clay bentonite panels, clay mats, etc. ("GCLs") were developed. GCLs generally fall into two main categories. In the first category a water-swellable colloidal clay, e.g., bentonite, is sandwiched between two geotextiles (examples of such products include Bentofix.RTM. manufactured by Naue Fasertechnik/Albarrie-Naue, Ltd and distributed by National Seal Co., Aurora, Ill., Bentomat.RTM. by Colloid Environmental Technologies, Colo., Arlington Heights, Ill., NaBento.RTM. manufactured by Huesker, Inc. of Charlotte, N.C. and Claymax.RTM. by the James Clem Corp., Fairmont, Ga.). In the second category of GCLs, bentonite is mixed with an adhesive and glued to a geomembrane (an example of such a product is Gundseal.RTM. produced by GSE Lining Technology, Inc., Houston Tex.). Additional GCL manufacturers include Environmental Protection Systems of Houston, Tex. and Environmental Protection, Inc. of Mancelona, Mich.
GCLs contain approximately 5 kg/m.sup.2 (1 lb./ft.sup.2) of bentonite and are manufactured in panels with widths of approximately 2 to 3 meters and lengths of 25 to 60 meters. The panels are placed on rolls at the factory where they are stored until shipped to the waste containment facility site where they are unrolled and installed in their final location. Their cost is substantially lower than that of compacted clay liners, i.e., thirty to sixty cents per square foot versus $3-10 per square foot for the compacted clay liners.
Although GCLs are less expensive and easier to install (due to their reduced bulk and prefabricated construction) than the compacted clay liners, they nevertheless also exhibit significant disadvantages. As noted above, the clay used in GCLs is typically bentonite, which exhibits a hydraulic conductivity of less than 10.sup.-7 cm/sec., but only when hydrated. Unhydrated bentonite, on the other hand, exhibits a hydraulic conductivity that is greater than the required value of 10.sup.-7 cm/sec. Thus, hydration is required to maintain impermeability but leads, as discussed below, to a loss of internal and interface (e.g., between two adjacent geosynthetics) strength, rendering such products particularly susceptible to damage due to shear caused, for example, by installation upon uneven (i.e., sloped) surfaces.
Further to the above, a significant disadvantage of GCLs is their low internal strength, i.e., at the interface between the bentonite and the geotextile or geomembrane, resulting from the hydration of the bentonite, which is of particular importance in areas prone to seismic activity. The peak and residual shear strength of hydrated bentonite correspond to a slope stability of 8 and 5 degrees, respectively. Thus, a hydrated bentonite GCL which is installed on ground having a slope greater than about 5-8 degrees will not be stable. Therefore, such prior art GCLs are susceptible to shear damage caused by sliding through, i.e., within, the internal bentonite filling. Prior art GCLs are also susceptible to shear damage caused by sliding at the top or bottom of the product because hydrated bentonite extrudes through the geotextile, causing a reduction in interface strength between adjacent geosynthetics.
Typical waste containment facility slopes range, however, from about 14 to about 26 degrees, with some proposed slopes of about 90 degrees. Thus static and seismic instability is a serious consideration in GCLs utilized in such applications once the bentonite hydrates. As a result, modifications, i.e., by the addition of one or more geomembranes emplaced above and/or below the GCL, are required to decrease and preferably prevent hydration. However, this creates additional interfaces, e.g., geomembrane/bentonite, along which shear failure can occur.
The earliest GCL products were known simply as GCLs since they consisted merely of a layer of bentonite sandwiched between two geotextiles. Subsequently, to increase the shear resistance of the bentonite, manufacturers began using vertical needle punched fibers to sew the geotextiles together in order to confine and strengthen the bentonite. This method is used in the Bentofix.RTM. and Bentomat.RTM. products marketed, respectively, by National Seal Company and Colloid Environmental Technologies Company. Another method known in the art is to stitch bond the geotextiles together. This method is used in the Claymax.RTM. and NaBento.RTM. products marketed by James Clem Corporation and Huesker, Inc., respectively. Such needle punched and stitched products are known as strengthened or improved GCLs. The vertical needle punching and stitch bonding also provides some additional shearing resistance in the middle of the GCL in an effort to prevent internal failure of the bentonite.
The strengthened construction described above suffers, however, from at least one significant drawback in that the vertical needle punching tends to tear or pull out due to small shear displacements (e.g., caused by shearing of the bentonite within the GCL), unconfined swelling of the bentonite, which may result in internal failure, (i.e., failure through the bentonite), or shear displacement along the upper or lower interface of the strengthened GCL. It has also been demonstrated that the stitching tends to act as a wick, thus increasing the permeability of the product. The shear displacement required to tear or pull out the vertical stitching is less than one inch, which can occur during use of such products in the field. Thus, strengthened GCLs provide only a minimal increase in internal strength over earlier GCLs known and used in the art. In fact, it has been demonstrated that the long-term internal strength in a strengthened GCL is approximately equal to the shear strength of bentonite alone due to the vertical stitching tearing or pulling out of the geotextiles under sustained shear stress.
For all the reasons set forth above, there has been a long felt need by those working in this field for a fluid barrier member which is stable when installed at inclinations greater than 5-8 degrees and which will not undergo internal failure upon hydration. As explained below, the stabilized fluid barrier member of the present invention meets all of these criteria.